This invention pertains generally to radar systems, and particularly to monopulse receivers of such radar systems.
It is known in the art that a monopulse receiver is used to produce, in response to radar return signals from an illuminated target, target tracking signals. More particularly, in response to such radar return signals, the radar receiver produces a sum signal and a pair of difference signals. One of the pair of difference signals is an azimuth difference signal and the other one of the pair of difference signals is an elevation difference signal. The pair of difference signals are normalized with the sum signal to produce a pair of tracking signals. One of the pair of tracking signals is an azimuth error signal and another one of the pair of tracking signals is an elevation error signal. These tracking signals are also referred to as an azimuth boresight error signal and an elevation boresight error signal.
As it is also known in the art, the sum signal together with the pair of difference signals may be sometimes multiplexed into a single channel, while in other cases a two-channel arrangement is used wherein the pair of difference signals are multiplexed together to produce a multiplexed pair of difference signals. In one of such channels, the multiplexed pair of difference signals are added with the sum signal and in another one of such channels, the multiplexed pair of difference signals are subtracted from the sum signal. In a monopulse receiver of the two-channel type described hereinabove, a so-called "dot-product angle-error-detector circuit" is useful to derive the azimuth and elevation error signals. If the sum signal is combined with the multiplexed pair of difference signals to produce composite signals (S+D) and (S-D) (where S is the monopulse sum signal and D is the multiplexed pair of difference signals), such composite signals may be downshifted to an appropriate I.F. amplifier and applied to a dot-product angle error detector circuit, or equivalent, finally to derive a desired angle error of a target.
Although known single channel or two-channel monopulse receivers may be useful in many circumstances, neither is satisfactory in all applications. For example, when a monopulse receiver is constructed with fewer than the conventional three I.F. signals, the sum and difference signals are combined by some means so that they may be individually retrieved at the output. Thus, the single or two-channel receiver may suffer from cross-coupling between the elevation and azimuth error signals, with a result that the accuracy of a correct determination of the angle error of any target is reduced. With the two-channel I.F. amplifier, satisfactory operation requires that amplitude and phase imbalances between the two channels be limited to very low amounts.
One technique used for reducing imbalances between the two channels is described in patent application, Ser. No. 382,971 having a filing date of July 10, 1989, and entitled "Monopulse Tracking Apparatus and Method of Operating Same" (which patent application is assigned to the same assignee as this application and is incorporated herein by reference). Although useful in many applications for correcting imbalances between the two channels, in other applications such a technique may not provide the requisite correction for amplitude and phase imbalances of a monopulse antenna and for errors introduced by monopulse arithmetic circuitry used to provide the azimuth difference signal and the elevation difference signal which, in turn, cause errors in the tracking signals. Also unacceptable amplitude and phase imbalances between the elevation error signal and the azimuth error signal can be introduced by the circuitry employed to multiplex the azimuth difference signal with the elevation difference signal.